The invention relates to a method for operating a reciprocating internal combustion engine.
Such a method of operating a reciprocating internal combustion engine in an engine braking mode of operation is known from U.S. Pat. No. 4,592,319. In the engine braking mode of operation, the reciprocating internal combustion engine is used as a brake, that is, as an engine brake, for example, for braking a motor vehicle. When driving downhill, for example, the reciprocating internal combustion engine is used during the engine braking mode of operation to at least substantially maintain a constant speed of the motor vehicle or to prevent the speed of the motor vehicle from increasing excessively. By using the reciprocating internal combustion engine as an engine brake, a service brake of the motor vehicle can be preserved. In other words, due to the use of the reciprocating internal combustion engine as an engine brake, the application of the service brake can be avoided or kept low.
To this end, in the method it is provided that the reciprocating internal combustion engine is used or operated as a decompression brake. In other words, the reciprocating internal combustion engine is operated in the engine braking mode of operation in the manner of a decompression brake, which is well-known from the general prior art. As part of the engine braking mode of operation, at least one exhaust valve of at least one combustion chamber in the form of a cylinder of the reciprocating internal combustion engine is closed for the first time within a working cycle. As a result, by means of a cylinder piston, gas, such as fresh air, present in the cylinder, may be compressed. Following the first closing, the exhaust valve is opened so that the air compressed by the piston is vented from the cylinder particularly in an abrupt way. By this discharge of the compressed air, the energy stored in the compressed air, which was transmitted by the piston, can no longer be used to move the piston from its top dead center to its bottom dead center or assist in such a movement. In other words, the compression energy is drained at least for the most part unused out of the cylinder. Since the piston or the reciprocating internal combustion engine has to expend work to compress the gas in the cylinder, which work cannot be used for moving the piston from the top dead center to the lower dead center as a result of opening of the exhaust valve, the motor vehicle can be braked.
The first or initial opening of the exhaust valve is followed by a second closing. In other words, the exhaust valve is closed a second time after the first opening. Therefore, gas still present in the cylinder may be compressed again by the piston. After the second closing, the exhaust valve is opened for a second time, so that the compressed gas may also be discharged a second time from the cylinder, without the compression energy stored in the gas being exploited for moving the piston from its top dead center to its bottom dead center. This at least double opening and closing is performed within a working cycle and allows the discharging of the compressed gas in the cylinder by the piston of the same cylinder.
The piston is pivotally coupled via a connecting rod to a crankshaft of the reciprocating internal combustion engine. The piston is translationally movable relative to the cylinder within the cylinder, wherein the piston moves from its bottom dead center to its top dead center. As a result of the pivoting coupling with the crankshaft, translational movements of the piston are converted into a rotational movement of the crankshaft, so that the crankshaft rotates about a rotational axis. A “working cycle” in a four-stroke engine has exactly two complete revolutions of the crankshaft. This means that one cycle of the crankshaft includes exactly a crank angle of 720 degrees. Within this 720-degree crank angle (°CA) the piston moves twice at its top dead center and twice at its bottom dead center. In a two-stroke engine, the “working cycle” is exactly one revolution of the crankshaft, i.e., a 360-degree crank angle (°CA).
The engine braking mode of operation differs in particular from a normal operation in that the reciprocating internal combustion engine is operated in the engine braking mode of operation without fuel injection, in which the reciprocating internal combustion engine is driven by wheels of the motor vehicle. In normal operation, however, the reciprocating internal combustion engine is operated in a so-called traction mode in which the wheels are driven by the reciprocating internal combustion engine. Moreover, in the normal mode of operation, a fired mode is used, in which not only air but also fuel is introduced into the cylinder. This results in (in the normal operation mode) a fuel-air mixture which is ignited and burnt.
In the engine braking mode of operation, however, no fuel is introduced into the cylinder, so that the reciprocating piston combustion engine in the engine braking mode of operation is operated in an unfired condition.
The object of the present invention is therefore to develop a method of the aforementioned kind such that a particularly high braking performance can be realized.
In order to develop a method such that a particularly high braking performance can be achieved in the engine braking mode of operation, according to the invention, the exhaust valve is kept open after the first opening and before the second closing, until the cylinder is filled with gas, which flows in particular on an exhaust side of the reciprocating internal combustion engine via at least one exhaust channel from at least one second cylinder, which is different from the first cylinder, of the reciprocating internal combustion engine. In other words, the invention proposes to introduce gas from at least one second cylinder into the first cylinder and thereby charge the first cylinder with the gas from the second cylinder. Thereby at least a so-called reverse charging can be performed, after a first decompression cycle of the first cylinder. The exhaust valve of the first cylinder then closes in time for the second time, so that the gas now present in the first cylinder and originating from the second cylinder is compressed by the piston of the first cylinder. Then, the exhaust valve of the first cylinder can be opened for the second time, so that the first cylinder performs a second decompression cycle and the energy stored in the compressed gas cannot be used for returning the piston of the first cylinder from its top dead center to its bottom dead center.
The exhaust valve of the first cylinder therefore performs, within a working cycle, at least two successive decompression strokes, whereby the two decompression cycles of the first cylinder are performed. The second decompression cycle is charged twice or multiple times, since during the second decompression cycle, the gas from the second cylinder is in the first cylinder. Due to this charging of the second decompression cycle, a particularly high engine brake power may be provided in the engine braking mode. The second decompression cycle or stroke may be provided so that the pressure in the first cylinder cannot surpass the value, against which at least one intake valve of the first cylinder can permanently open.
With respect to conventional valve controls in four-stroke engines in the engine brake mode, a considerable increase of engine braking power can be provided by the inventive method, in particular in a lower speed range.
A further embodiment is characterized in that in the engine braking mode, within a working cycle, at least one second exhaust valve of the second cylinder is closed for a first time, then opened for a first time, then closed for a second time and then opened for a second time, in order to discharge gas compressed in the second cylinder from the second cylinder by means of a second piston of the second cylinder. This means that the second cylinder or the second exhaust valve of the second cylinder is operated like the first cylinder or the first exhaust valve of the first cylinder.
In this case, the first cylinder is filled with at least a portion of the gas discharged by the second cylinder, while the second exhaust valve of the second cylinder, after its second opening and before its first closing or after its first opening and before its second closing, is at least partially open. Due to the fact that the second exhaust valve and the first exhaust valve are at least partially open, the gas compressed by the second piston may vent on the discharge or exhaust side of the reciprocating internal combustion engine from the second cylinder and may flow via at least one exhaust channel of the first cylinder into the first cylinder. In this way, a decompression cycle or a decompression stroke of the second cylinder or of the second exhaust valve is used for charging the first cylinder for its second decompression cycle. Due to this charging, a particularly high air quantity is provided in the first cylinder by its second decompression stroke, therefore providing a particularly high engine braking power.
A particularly high charging of the first cylinder may be accomplished by the fact that the exhaust valve of the first cylinder, after the first opening and before the second closing, is kept open, until the first cylinder is filled with respective gas, which flows from the second cylinder, on the exhaust side, through at least a respective exhaust channel, and from at least one third cylinder of the reciprocating internal combustion engine. This means that the first cylinder is charged with gas not only from the second cylinder, but also with gas from the third cylinder, so that a particularly high engine braking power is achieved.
In a further advantageous embodiment of the invention, it is provided that in the engine braking mode, within a working cycle, at least a second exhaust valve of the second cylinder is closed for a first time, then is opened for a first time, then is closed for a second time and then is opened for a second time, in order to discharge compressed gas from the second cylinder by means of a second piston of the second cylinder. As already noted, it is provided that the second cylinder and its second exhaust valve are operated like the first cylinder and the first exhaust valve. Moreover, it is provided that in the engine braking mode, within a working cycle, at least a third exhaust valve of the third cylinder is closed for a first time, then is opened for a first time, then is closed for a second time and then is opened for a second time, in order to discharge compressed gas from the third cylinder, by means of a third piston of the third cylinder. This means that also the third cylinder and its third exhaust valve are operated like the first cylinder and the first exhaust valve. In this way, a decompression brake is provided by the three cylinders, so that a particularly high engine braking power is achieved.
The first cylinder is filled with at least a portion of the gas discharged by the second cylinder, while the second exhaust valve, after its second opening, and before its first closing, is open. Moreover, the first cylinder is filled with at least a portion of the gas discharged by the third cylinder, while the third exhaust valve, after its first opening and before its second closing, is at least partially open. It is therefore provided that the second decompression cycle of the second cylinder and the first decompression cycle of the third cylinder are used for charging the first cylinder for its second decompression cycle. Thereby, during the second decompression cycle, a particularly high quantity of air is present in the first cylinder, so that a particularly high engine braking power is achieved.
It is also contemplated, that, for example, the first cylinder for its first decompression cycle, is filled with gas formed by fresh air through at least one intake channel. An intake valve associated with the intake channel is at least in its open position, so that, by moving the piston of the first cylinder from the top dead center to the bottom dead center, gas of fresh air is sucked into the first cylinder. This fresh air may then be compressed in the first decompression cycle by the first piston. The compressed fresh air flows, after the first decompression cycle, from the first cylinder. For the second decompression cycle, the first cylinder is filled with gas, which originates from the second decompression cycle of the second cylinder and from the first decompression cycle of the third cylinder.
The respective gas may flow on the exhaust side of the reciprocating internal combustion engine through at least a respective exhaust channel from the second cylinder and from the third cylinder, and into the first cylinder, through at least one exhaust channel of the first cylinder.
To this end, the three cylinders are connected fluidically to one another for example via an exhaust manifold, which is arranged on the exhaust side and serves for guiding exhaust gas or gas flowing out of the cylinders. At an instant, at which the three exhaust valves of the three cylinders are open, the three cylinders are connected via the exhaust manifold fluidically with each other, such that the described transition of the gas from the second cylinder and the third cylinder into the first cylinder can take place.
Another embodiment is characterized in that the exhaust valve of the first cylinder is held open after the first opening, at least up to 210 degrees of crank angle after top dead center, especially after ignition top dead center, of the piston of the first cylinder. The ignition top dead center of the first piston is the top dead center of the piston, in whose area, in firing operation of the reciprocating internal combustion engine, the ignition of the fuel-air mixture takes place. This ignition is obviously absent in the engine braking mode of operation, wherein the term “ignition top dead center” is merely used to distinguish this ignition top dead center from the top charge exchange dead center (TD) which is reached by the first piston during ejection of exhaust gas from the first cylinder.
Because the exhaust valve of the first cylinder is kept open up to at least 210 degrees of crank angle after top dead center, the first cylinder can be charged with a particularly high amount of gas, so that a particularly high engine braking power can be realized.
Especially advantageous is the case where the exhaust valves in the engine braking mode of operation travel less than in a normal mode of operation, different from the engine braking mode of operation, in particular traction, of the reciprocating internal combustion engine. This means that in the engine braking mode of operation, the exhaust valves are not opened at full stroke as in normal operation (fired or combustion mode). This full stroke is absent in the engine braking mode of operation. Rather, the exhaust valve is opened with a comparatively smaller stroke, both in the first opening and the second opening. It can be provided that the strokes during the first opening and the second opening are the same, or that the exhaust valve of the first cylinder during the first opening and the second opening opens with different strokes.
The invention also includes a reciprocating internal combustion engine of a motor vehicle, which is designed for performing a method according to the invention. Advantageous embodiments of the method according to the invention are to be regarded as advantageous embodiments of the reciprocating internal combustion engine and vice versa.
Further advantages, features and details of the invention will become apparent from the following description of embodiments and from the drawings. The above features and feature combinations mentioned in description, and those features and feature combinations mentioned below in the description of the figures and/or shown in the figures may be used not only in the particular combination indicated, but also in other combinations or alone, without leaving the scope of the invention.